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The present study investigated the application of zero valent iron to remediate the arsenic in naturally contaminated groundwater. A performance evaluation was conducted in the laboratory on groundwater contaminated with artiˆcial arsenic using sodium arsenate (Na2HAsO4. 7H2O) to simulate the arsenic concentration in the groundwater. Batch and column experiments were performed to evaluate the arsenic removal capacity by zero valent iron and the removal mechanism. The‰ ow rate (up-‰ ow mode) was maintained for 180 days in each column. The results from both the batch and the column experiments showed that more than 99z of the arsenic was removed successfully. In the column experiments, the arsenic was e‹ ciently removed and the arsenic concentration in the treated water decreased to below the limit of 10 mg/L (WHO's standard) even when the columns were packed with only 25z ZVI by volume. We used SEM and XRD to characterize the surface morphology and the corrosion layer which formed on pristine ZVI and arsenic-treated ZVI to elucidate the arsenic removal mechanism. XRD and SEM results revealed that ZVI gradually converted to a magnetite/maghemite corrosion product mixed with lepidocrocite. Adsorption followed by co-precipitation was an important pathway to removing the arsenic by ZVI. Our results suggest that ZVI, combined with sand, is a suitable candidate for the ex-situ treatment of groundwater in the neutral pH range and in the presence of dissolved oxygen.